Personally I would advise against it.
First off they didn’t test Harvoni while using every possible drug out there and especially not with an illegal drug like marijuana. So there is no data about using them in combination. Basically you will be making yourself a test subject.
Some doctors, insurances, or pharmacy benefits providers may not allow you to be prescribed treatment while using illegal drugs they might deny you treatment.
Your liver has to process everything you take into your body. Your liver is already under attack from hepatitis c and you are adding to the liver’s burden by introducing a drug into your blood stream that your liver has to detoxify while already fighting your hep c infection. This becomes even more serious if you already have liver damage. There is debate on the safety of marijuana for the liver. There is some evidence marijuana may lead to faster liver scarring.
Also as you said you would have a hard time stopping marijuana that is probably the most important reason to quit using marijuana. Do you want to have a drug running your life and dictation your actions? Who do you want to be in charge of your life and your body you and your mind, your hopes, desires and life goals? Or do you want a plant running your life and your decisions?
The best person to ask this question is your personal physician that prescribed Harvoni. If unwilling to discuss this with your doctor that should be an answer as well. If you think your doctor would say no that should answer your question as well. If you feel guilty of think it is wrong it probably is.
My suggestion is to quit using marijuana. No good will come to you for continuing to use and there are several ways your continued use could be harmful.
Best of luck on treatment and in life